Generally, FT synthesis is carried out to produce hydrocarbons of all states ranging from a gas fraction to a solid fraction by using a carbon monoxide-hydrogen mixture gas (synthesis gas) as a raw material, and the product thereof has normal paraffins as a main component. By the FT synthesis, a clean fuel oil that without a sulfur component, a nitrogen component or an aromatic component can be prepared. The FT product has, as a main component, normal paraffins having a wide carbon number distribution of 1 to about 100. Accordingly, in order to prepare a diesel fuel oil, a middle fraction having a carbon number of about 10 to 21 is subjected to hydroisomerization or a wax fraction having a carbon number of 22 or more which is a solid fraction at a room temperature is hydrocracked to produce a middle fraction, thereby producing the diesel fuel oil. In this manner, generally, the product produced by the FT synthesis is converted into a fuel oil which is clean and which has excellent cold flow properties by way of an upgrading process such as hydroisomerization or hydrocracking.
Between an FT synthesis reactor and a reactor for each upgrading process, a buffer tank for storing a wax fraction or a middle fraction which is obtained in the FT synthesis process and is used as a raw material for the upgrading process is installed to deal with starting the process, modifying operation conditions thereof (e.g. increasing or decreasing the charging amount of raw material), or the occurrence of errors in the process.
The wax fraction in the FT synthesis product has, as a main component, normal paraffins having a carbon number of about 20 to 100, as described above. In addition, the wax fraction is solid and has no flowability at a room temperature. Accordingly, generally, a process line through which the wax fraction flows is a steam tracing line, or tank heating is performed to maintain the flowability.
However, a small amount of impurities such as oxygenated compounds or olefins is contained in the wax fraction although the amount of impurities varies depending on FT reaction conditions and the type of FT catalyst used. For this reason, when the wax fraction is stored in a tank for a certain period of time, a polymeric substance such as a peroxide or gum component may be produced depending on the storage conditions of the wax fraction, and such products may have an influence on activity of a catalyst in the hydrocracking process. Meanwhile, methods or apparatuses for removing the impurities in the wax fraction and a technique of suppressing oxidation of FT products have been studied (for example, Patent documents 1 and 2).
Patent document 1: Published Japanese Translation No. 2005-502739 of the PCT International Application
Patent document 2: Published Japanese Translation No. 2004-534881 of the PCT International Application